There are many applications where a video camera may be used, where the target being observed is not oriented such that it is “upright”, i.e. earth normal, in the video scene that is being presented on a monitoring or recording device. Such is often the case in video inspection systems, where a camera is used to look inside of an opening, and the camera may rotate during its traversal towards the observation point. It is also the case where a fixed orientation camera is used to observe variable orientation objects—such as a camera observing a manufacturing process that produces items with variable orientation (such as text printed on bottle caps, which are affixed to bottles, and are being monitored as they roll by). It is further often desirable to orient the video so that it more closely approximates “upright” and normal viewing conditions. This can help to visually orient the viewer, and can help prevent neck strain from having to cock the head to the side to bring the image closer to normal.
Video pipe inspection systems are commonly used to inspect sewer, water and well pipes for blockages and defects. Electrical conduits and other long narrow passages may be similarly inspected. Typically a resilient flexible push cable is pushed down the pipe. A rugged camera head connected to the distal or remote end of the push cable receives power through the push cable. A video signal from an internally mounted video camera sends NTSC or other video signals back through the push cable for display and recording. The camera head is usually centered inside the pipe by radially extending brushes or fins. Alternatively, the camera head may be supported by small wheels that roll along the interior of the pipe. See for example U.S. Pat. No. 6,545,704 B1 of Mark S. Olsson et al. assigned to Deep Sea Power & Light Company of San Diego, Calif.
As the camera head snakes its way through the pipe it usually rotates as the push cable twists and turns. The video image follows this motion. Users would prefer a video image that maintains its frame of reference so that the location and nature of defects can be more easily recognized. However, since the camera head is inaccessible after it has been pushed down the pipe, it cannot be manually righted. In some cases, water may be present in the bottom of the pipe, in which case the viewer has a frame of reference, but even then, the periodic twisting of the video image as the camera head moves along the pipe can be very tedious and annoying. Furthermore, water is not always present, or it may fill the pipe entirely. In either of these cases, there is no frame of reference to tell the viewer which part is the top of the pipe.
U.S. Pat. No. 4,372,658 of O'Connor et al. discloses a pipe inspection apparatus in which a camera is supported for rotation by ball bearings mounted within a wheeled housing and a weight is used to orient the camera. Slip rings are also provided within the housing for transmission of electrical control signals to the camera. This design is not compact and rugged enough for use by plumbers.
UK patent application GB 2 342 419 A filed in the name of Pearpoint Limited on Oct. 5, 1998 and published Apr. 12, 2000 discloses a camera head for pipe inspection in which forward looking and sideways looking cameras are mounted within a rotatable member suspended between bearings at opposite ends of a casing. A motor rotates the member to compensate for motion in the forward looking view. An operator controls the view obtained from the camera head with a keypad and joystick which provides the control signals to the camera head. A commercial version of this camera head incorporates “auto uprighting” of the cameras. The Pearpoint camera head is complex, expensive to manufacture and subject to failures since it lacks the ruggedness required in many pipe inspection applications. The motorized mechanisms take up substantial space inside the camera head, thereby making it difficult to downsize the camera head for inspecting small pipes. Power and control signals must be sent to the motor, requiring extra conductors in the push cable.
U.S. Pat. No. 6,611,661 of Buck discloses a camera head for pipe inspection in which the camera body is mounted for free rotation within a camera housing and a leveling weight made of tungsten or lead is physically attached to the camera body in a permanent fashion. The center of mass of the weight is displaced from the axis of rotation of the camera body so that the camera body is leveled via gravitational forces. A bearing is positioned between the camera body and the camera housing. A slip ring has portions that fit on inner and outer races of the bearing. However, this design does not lend itself to easy removal and/or repair of the video camera and associated electronics within the camera head.
In the past, there have also been electronic solutions to the problem of orienting a video image from a remote video camera. If one is only interested in a rotation of one hundred and eighty degrees (the coarsest rotation—commonly called a screen flip), this can easily be done in one of two ways. The video transmitted by the camera can be converted it to a digital format and re-mapped so that it is presented with what was originally the lower, right most pixel, remapped to the upper left most corner, and so on. The remapped digital data can then be reconverted to analog form. Alternatively in the case of a monitor having a cathode ray tube, the vertical and horizontal gun polarity can be reversed. Instead of scanning from left to right, top to bottom, the guns scan right to left, bottom to top. Either approach yields the same effect of rotating the video from the camera by one hundred and eighty degrees.
A digital flip and/or mirror is also commonly used both in LCD monitors as well as in some CCD cameras. One advantage of doing the flip before recording is that the corrected image is then recorded. Pipe inspection systems in use today that invert the image on the monitor do not allow the inverted image to be recorded. The main advantages of the flip approach are low cost and the fact that it preserves the original aspect ratio of the video (typically 4:3). The primary disadvantage is the limited rotational resolution (only offering two positions—0 degrees and 180 degrees of rotation).
Some manufacturers of video equipment have taken a video stream, converted it into a digital format, performed a matrix operation on the digital data to rotate the entire image by a predetermined amount, and then re-converted the digital date to an analog signal. This approach is optimal in terms of the rotational resolution, however it is extremely computationally intense, and therefore requires a significant cost in parts and power. It also suffers from the drawback that the rectangular 4:3 array is clipped so that some video content is lost at any angles other than zero and one hundred and eighty degrees. At rotations of ninety and two hundred and seventy degrees, the entire right and left lobes of the source video are lost.